Cohesion between sister chromatids, as well as tension created by the pull of kinetochore microtubules, is essential to ensure efficient separation of chromatids at mitotic anaphase or in meiotic anaphase II. Explain why sister chromatid cohesion is important, and discuss the role of the proteins cohesin and separase in sister chromatid separation.

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Sister chromatid cohesion is important because it ensures that the replicated chromatids remain attached to each other until the appropriate stage of cell division. This cohesion prevents premature separation, which could lead to unequal distribution of genetic material and aneuploidy.

The protein complex cohesin plays a critical role in maintaining sister chromatid cohesion. Cohesin forms a ring-like structure that encircles the sister chromatids, physically holding them together after DNA replication in the S phase of the cell cycle.

During the metaphase stage of mitosis or meiosis II, tension is created by the pull of kinetochore microtubules on the sister chromatids. This tension is essential for proper alignment of chromosomes at the metaphase plate and ensures that chromatids are correctly oriented for separation.

The enzyme separase is activated at the onset of anaphase. Separase cleaves a subunit of the cohesin complex, specifically the SCC1 (also known as RAD21) subunit, which leads to the release of sister chromatids from each other.

Once the cohesin complex is cleaved by separase, the sister chromatids are free to move toward opposite poles of the cell, driven by the shortening of kinetochore microtubules. This ensures accurate segregation of genetic material into the daughter cells.

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Key Concepts

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Sister Chromatid Cohesion

Sister chromatid cohesion refers to the physical connection between sister chromatids, which are identical copies of a chromosome formed during DNA replication. This cohesion is crucial during cell division, as it ensures that the chromatids remain attached until the appropriate stage of mitosis or meiosis, preventing premature separation and ensuring accurate distribution of genetic material to daughter cells.

Cohesin Proteins

Cohesin is a protein complex that plays a vital role in maintaining sister chromatid cohesion. It encircles the chromatids, holding them together from the time of DNA replication until anaphase. The regulated release of cohesin at the onset of anaphase is essential for allowing the chromatids to separate and move to opposite poles of the cell, ensuring proper chromosome segregation.

Separase

Separase is an enzyme that is crucial for the separation of sister chromatids during cell division. It cleaves the cohesin proteins, thereby releasing the tension that holds the chromatids together. The activation of separase is tightly regulated to ensure that it only occurs at the correct stage of cell division, preventing errors in chromosome segregation that could lead to aneuploidy.

Related Practice

Textbook Question

The diploid number of the hypothetical animal Geneticus introductus is 2n = 36. Each diploid nucleus contains 3 ng of DNA in G₁.

What amount of DNA is contained in each nucleus at the end of the S phase?

Textbook Question

The diploid number of the hypothetical animal Geneticus introductus is 2n = 36. Each diploid nucleus contains 3 ng of DNA in G₁.

Explain why a somatic cell of Geneticus introductus has the same number of chromosomes and the same amount of DNA at the beginning of mitotic prophase as one of these cells does at the beginning of prophase I of meiosis.

Textbook Question

For the retinal cancer retinoblastoma, the inheritance of one mutated copy of RB1 from one of the parents is often referred to as a mutation that produces a 'dominant predisposition to cancer.' This means that the first mutation does not produce cancer but makes it very likely that cancer will develop.

What is the genotype of a normal cell in the retina in a person who has sporadic retinoblastoma? What is the normal cell genotype if the person has hereditary retinoblastoma? Explain the reason for the difference between the genotypes.

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Textbook Question

Using RB1⁺ for the normal wild-type allele and RB1⁻ for the mutant allele, identify the genotype of a cell in a retinoblastoma tumor.

Textbook Question

In a test of his chromosome theory of heredity, Morgan crossed a female Drosophila with red eyes to a male with white eyes. The females were produced from Cross A, shown in the Figure below. Predict the offspring Morgan would have expected under his hypothesis that the gene for eye color is on the X chromosome in fruit flies.

Textbook Question

The diploid number of the hypothetical animal Geneticus introductus is 2n = 36. Each diploid nucleus contains 3 ng of DNA in G₁.

Complete the following table by entering the number of chromosomes and amount of DNA present per cell at the end of each stage listed.